Singlet molecular oxygen in the form of the delta state, DeltaO2 has been identified as a potential atmospheric oxidant. Although the ambient levels of DeltaO2 are not well established it is sufficiently long lived to react with atmosphere pollutants and/or if inhaled to reach the alveolar region of the lung. In the atmosphere it can react with olefins and polycyclic aromatic hydrocarbons to form toxic and possibly mutagenic compounds which would be harmful to human health if injected. It is also possible that singlet oxygen may form in situ by a number of mechanisms, e.g., the reaction of ozone with biological substrates. In the lung singlet oxygen is an agent that can produce subtle chemical alterations of the pulmonary surfactant, alveolar macrophages and the pulmonary epithelium which can predispose to various pathological conditions. In this respect it is likely that singlet delta oxygen will attack the unsaturated fatty acid moieties of the alveolar phospholipids forming hydroperoxides via the "ene" reaction. Among the possible oxidation products to be expected is malonyl aldehyde which has recently been reported to exhibit carcinogenic activity. Also, the hydroperoxides are known toxins that exert their effects on the lungs causing inflammation, edema and hemorrhaging. The formation of hydroperoxides in the lung and the mechanism by which they act on the tissue of the lung are at the present time unreported. They may give rise to lipid epoxides. These substances can act aklylating agents and may be ultimately be responsible for mutagenesis or carcinogenesis. Accordingly research program is proposed to determine the role of singlet delta oxygen in environmental pollution and further its impact on environmental health.